1. Field of the Invention
The present invention relates to a mounting pad structure of wire-bonding type lead-frame packages. More particularly, the present invention relates to a mounting pad structure which can improve the performance of wire-bonding type lead-frame packages.
2. Description of the Related Art
Modern electronic products typically enclose a semiconductor chip and a carrier for electrically connecting with the semiconductor chip. At present, there are three major techniques for connecting a chip to a carrier, namely, a wire-bonding process, a flip-chip process and a tape-automated-bonding (TAB) process. If the carrier is a lead frame, the wire-bonding process is often used to connect the chip with the leads on the lead frame. Bonding wires of a package induces parasitic inductance which becomes more significant when the operating frequency is beyond several giga-hertz (GHz) for Radio Frequency (RF) products. The parasitic inductance has adverse effects on the circuit performance. Therefore, a method for reducing the parasitic inductance and improving the frequency response at high frequencies shall be introduced.
FIG. 1A is a schematic cross-sectional view of a conventional QFN (Quad Flat Non-Lead) package mounted on a circuit board for RF applications. FIGS. 1B and 1C are perspective view and top view respectively of the coplanar bonding wire structure of single-ended mode shown in FIG. 1A. As shown in FIGS. 1A, 1B and 1C, the chip package 100 comprises a chip 110, a lead frame 120, a plurality of bonding wires 130 and an insulation material 140. The lead frame 120 has a die pad 121 and a plurality of ground leads 122 and signal leads 123. The ground leads 122 and signal leads 123 are distributed evenly at the peripheral area of the die pad 121. The chip 110 is attached to the die pad 121 through adhesive glue. The bonding wires 130 connecting the chip 110 to the respective leads 122, 123 are formed in a wire-bonding process. The die pad 121 and the ground lead 122 are electrically grounded to a ground plane 170 through a ground pad 181, a die pad landing and ground vias 160. The insulation material 140 encapsulates the chip 110, the lead frame 120 and the bonding wires 130. The die pad 121, the ground leads 122, and signal leads 123 of the chip package 100 are attached to the circuit board 190 through the application of adhesive materials.
FIG. 1D is a top view of the mounting pad structure of single-ended mode shown in FIG. 1A. In the conventional pad design, the signal pad 182 is underneath the signal lead 123 and has a shape which corresponds to the signal pad 182. Similarly for the ground pad 181, it is underneath the ground lead 122 and has a shape which corresponds to the ground pad 181. The ground pad 181 is connected to the die pad landing which is underneath the die pad 121 and is electrically grounded to a ground plane 170 through ground vias 160.
FIG. 2A is a perspective view of a conventional coplanar bonding wire of differential mode. FIG. 2B is a top view of a conventional coplanar bonding wire of differential mode. The differential mode involves one transmission line 231 carries a positive signal and the other transmission line 231 carries a negative signal for signal pins. The signals are equal in amplitude and opposite in polarity. The two transmission lines 231 have a differential impedance of around 100 Ohms. The two signal leads 223 are adjacent to each other in a differential-mode wire-bonding type package. To the opposite side which is adjacent to the other signal lead 223, there is an adjacent ground lead 222. Underneath the signal lead 223, there is a signal pad 282 and similarly, there is a ground pad 281 underneath the ground lead 222. The size and shape of the ground pad 281 and the signal pad 282 correspond to the ground lead 222 and the signal lead 223 respectively. The ground pad 281 is connected to the die pad landing which is underneath the die pad and is electrically grounded to a ground plane 170 through ground vias 160.